in transmission and high resolution electron microscopy, it is frequently observed in biotites and chlorites that there is in fact a mixture of elementary sheets of biotite and chlorite. According to common terminology, these crystals are biotite-chlorite interstratifications. In this article we will use the term “biochlorite” as an abbreviation for “biotite-chlorite interstratification”. The mixing energy of the sheets is close to 0, which confirms that ‘biochlorites’ can be stable crystals. The sequence of sheets is usually disordered, but the 1-1 ‘biochlorite’ (1 biotite-1 chlorite folio) is frequently observed. The “sequence of shifts” (shifts between successive tetraedric levels) is generally disorderly — with a trend towards type 1M ordered in pure biotite (i.e. in regions not containing chlorite sheets). Lateral transitions: biotite folio → chlorite folio is always of the following type: potassium ion plan → Brucite-type sheet. In these transitions, the two adjacent talc sheets are structurally unchanged, but they are — or not — shifted from each other (equivalent to a partial defect). Various methods of intercaling serpentine leaves in ‘biochlorites’ are also shown.